Abstract: To reveal the permeability surge mechanism and critical water-resisting thickness of the sandy mudstone roof in water-rich roadways after excavation disturbance, the No. 16191 working-face roadway of Zhaogu No. 1 Mine was taken as the engineering background. Field investigation, mineral composition analysis, mechanical tests under different water-content states, and FLAC3D fluid–solid coupled numerical simulation were conducted to analyze the water-induced deterioration characteristics of sandy mudstone, permeability evolution, and roof water-resisting capacity under different thickness conditions. The results show that roof seepage or dripping mainly occurs when the sandy mudstone thickness is 2.9–3.3 m, whereas dry roof conditions are generally observed when the thickness is 3.8–4.9 m. Compared with the non-seepage sections, roof subsidence and bed separation in the seepage sections increase by 40–60 mm and 50–75 mm, respectively. Sandy mudstone contains hydrophilic minerals such as montmorillonite and illite, and its strength and stiffness decrease significantly after water immersion. When the water content increases to 4.58%, the uniaxial compressive strength decreases from 21.2 MPa to 10.7 MPa, and the elastic modulus decreases from 4.14 GPa to 2.85 GPa. Numerical simulation results show that the high-permeability zone in the roof develops upward in an arch shape after roadway excavation, and the local permeability increases by 2–3 orders of magnitude compared with the initial state. When the sandy mudstone thickness is 3.0 m, the high-permeability zone tends to connect with the overlying sandstone aquifer. When the thickness reaches 3.5 m, the high-permeability zone does not penetrate the aquiclude, and the deformation, unloading zone, and plastic zone of the surrounding rock are significantly reduced. Therefore, the critical water-resisting thickness of the sandy mudstone roof under the studied engineering conditions is determined to be 3.5 m.